There are many devices today that utilize the technology of the global positioning system The global positioning system (GPS) is based on a constellation of twenty-four satellites orbiting around the earth that broadcast precise data signals. A single GPS receiver is capable of receiving these signals and can calculate its position (latitude and longitude), altitude, velocity, heading and precise time of day using data signals from at least four GPS satellites. Thus, these GPS receivers can locate themselves anywhere on the planet where a direct view of the GPS satellites is available.
Each satellite transmits two signals, an L1 signal and an L2 signal. The L1 signal is modulated with two pseudo-random noise codes, the protected code and the course/acquisition (C/A) code. Each satellite has its own unique pseudo-random noise code. Civilian navigation receivers only use the C/A code on the L1 frequency. In a positioning device that utilizes the GPS, a GPS receiver measures the time required for the signal to travel from the satellite to the receiver. This done by the GPS receiver generating a replica of the pseudo-random noise code transmitted by the satellite and precisely synchronizing the two codes to determine how long the satellite's code took to reach the GPS receiver. This process is carried out with at least four satellites so that any error in the calculation of position and time is minimized.
A positioning device utilizing GPS is an effective tool in finding a location or determining a position. However, a device utilizing GPS has many limitations. One significant limitation is that GPS is generally unsuitable for indoor positioning applications since a direct view of the GPS satellites is not available. Therefore, it is desirable to have an independent positioning system utilizing technology other than the GPS or working in conjunction with GPS that is functional indoors and in other locations where GPS is not functional.
Another problem with GPS systems is that many types of errors are inherent to the physics of transmission of the data signal from the GPS satellite down to the earth. For instance, ephemeric errors are errors caused by gravitational pulls on the satellites from the moon, sun, and the pressure of solar radiation. These errors affect the satellite's orbit. Other types of errors include ionospheric and tropospheric errors. A GPS receiver with this type of information may use this information to calculate position and time with greater accuracy. Therefore, it is desirable to have a positioning system utilizing technology in conjunction with GPS that supplies augmentation data to help GPS receivers correct errors that may occur in calculating time and location.